Amino acids are major primary metabolites. Their uptake, translocation, compartmentation, and re-mobilization require a diverse set of cellular transporters. Here, the broadly expressed gene product of
CATIONIC AMINO ACID TRANSPORTER 9 (
CAT9) was identified as mainly localized to vesicular membranes that are involved in vacuolar trafficking, including those of the trans-Golgi network. In order to probe whether and how these compartments are involved in
amino acid homeostasis, a loss-of-function cat9-1 mutant and ectopic over-expressor plants were isolated. Under restricted
nitrogen supply in soil, cat9-1 showed a chlorotic phenotype, which was reversed in the over-expressors. The total soluble
amino acid pools were affected in the mutants, but this was only significant under poor
nitrogen supply. Upon
nitrogen starvation, the soluble
amino acid leaf pools were lower in the over-expressor, compared with cat9-1. Over-expression generally affected total soluble
amino acid concentrations, slightly delayed development, and finally improved the survival upon severe
nitrogen starvation. The results potentially identify a novel function of vesicular
amino acid transport mediated by
CAT9 in the cellular
nitrogen-dependent
amino acid homeostasis.